The appearance of printed text and/or images on paper can be affected by the presence of a coating on the paper. The coating can contain a mixture of clay, pigment, and binder. When ink is applied to an uncoated paper it is absorbed by the paper. When ink is applied to coated paper it sits on the coating. This attribute allows ink printed on coated paper to retain a crisp edge. As a result, coated paper generally produces sharper, brighter images and has better reflectivity than uncoated paper.
Most absorbent material swells upon uptake of water giving them poor rheological performance in coating applications. In addition, most water swollen pigmented coatings shrink upon drying resulting in low gloss measurements. Rigid water absorbent materials circumvent the shrinkage but usually fail to provide glossy coatings upon calendering. Currently, high gloss ink jet coated paper is produced using expensive alternative coating technology such as cast coating. Ink-jet receptive coatings have been based on two technologies—swellable polymeric layers and microporous layers. As the speed of ink-jet printers has increased, the need for quick drying coatings has all but eliminated the use of swellable polymeric coatings. As ink-jet technology advances and begins to make inroads into the commercial and graphic arts arenas, the demand put on the microporous coating media will only intensify.
Microporous coating layers are currently formulated from silica and alumina pigments, a binder, and various functional additives. The primary objective of these receptive layers is to generate a maximum amount of fine pores near the surface in order to aid in quickly absorbing the ink. These coatings generate a porous structure that, in addition to facilitating rapid fluid absorption, also provide the necessary volumetric capacity to handle the high levels of ink applied to the sheet. Amorphous silica, in its many forms, is the predominate pigment used for microporous coatings. For matte-finished coatings silica gel has been the pigment of choice. Silica gel pigments for matte applications are typically in the 1-10 μm range of particle size. The high internal porosity of silica gels (up to 2.0 cm3/g), along with their nanometer-sized pores provide the driving force for liquid uptake.
Glossy coatings require the use of particles in the nanometer size range. Non-porous colloidal silica and fumed silica have been used with success in producing glossy, microporous coatings. Colloidal silica is a dispersion of non-porous silica particles in water. These primary, dense phase particles can range in size from 10 nm to over 100 nm. The porosity of coatings based on colloidal silica is due to the packing structure of the particles. Fumed silica is produced by the flame hydrolysis of SiCl4, and generates small primary particles in the range of 7 to 40 nm. These primary particles stick together via hydrogen bonding into micron sized aggregates. The porosity of ink-jet coatings based on fumed silica is due to the porous structure created by primary particles associating into secondary structures.
In addition to the small particle size silicas, fumed alumina is another pigment commonly used in the production of glossy inkjet papers. Alumina has the distinct advantage of being cationic at neutral pH, which provides a means of segregating anionic dyes and pigments from the ink near the coating surface. This inherent property of alumina may reduce the need for cationic fixatives in the coating formulation.
While silica and alumina pigments are effective once they have been applied to the substrate, they do pose production problems that limit their use to relatively slow coating speeds. The coating solids level is a limiting factor when using silica pigments due to rheology and water holding issues. The high water content of silica-based coatings create a high drying demand, which, taken with the rheological difficulties of these dispersions, limits their use to slow off-machine coaters. In addition, high gloss grades require special coating conditions such as cast coating.
Therefore, there exists a need to reduce the costs associated with the production of ink jetreceptive coated paper from both the raw materials and the coating application viewpoint. Silica and alumina pigments are expensive in relation to other pigments used in commercial printing (e.g., CaCO3 and kaolin), and there has been efforts to provide the industry with ink-jet optimized versions of these pigments. The major opportunity to decrease the cost associated with high quality, ink-jet papers is the opportunity to produce these grades on-machine with existing capabilities. A novel, high-glossing pigment that provides the rapid adsorption associated with silica pigments but is able to be coated at conventional high machine speeds would facilitate this opportunity tremendously.
As such, improvements in producing the different grades of coated paper and the coating compositions used to form the coated paper continues to be a desire in the art.